This invention relates to a method and composition for preventing for an extended period of time fouling of submersed or submerged objects or marine structures while also minimizing pollution and more particularly to a method and composition for preventing fouling and pollution for an extended period of time by using organotin compounds chemically combined in or bonded to polymers.
From the beginning of man's attempt to use water to travel, he has been plagued with the problem of his ships, buoys, pilings, or other objects placed in the water, being fouled by organisms present in the water. It has been found that microorganisms, their viscous bio-organic products and absorbed organic matter, constitute a tenacious, opaque slime which forms on these submerged surfaces. The initial organisms in this fouling sequence are bacteria followed by a biotic progression of diatoms, hydrids, algae, bryozoans, protozoans, and finally macrofoulants. Macrofoulants tend to be rugophilic, settling on roughened surfaces in preference to smooth surfaces. It is thought that primary marine slimes precondition the surface in some manner stimulating the settling of macrofoulants. This theory is supported by the fact that barnacle settlement is less frequent on clean glass surfaces compared to those covered with emollient films high in particulate matter. This film may provide a physical substrate and/or a nutritive source which encourages the attachment of macroscopic plants and animals.
The resultant effect of a concentration of these plants and animals settling and attaching themselves to ships are well known: they contribute significantly to speed reduction; they increase fuel consumption; and, in the area of concern of water craft detection, they strengthen the noise signature of vessels under way thereby rendering covert activity more difficult.
This problem of marine growth (fouling) applies not only to vessels but also to other submersed or submerged objects. For example, fouling of sonar domes has been found to seriously limit the active and passive modes of operation of ships' acoustical systems. Fouling of moored data systems and ship-and-shore facilities by marine organisms impedes operations and necessitates a large maintenance allocation. Buoys shift due to the excessive weight of fouling organisms. Effective operation of sonars is hampered by the self-noise generated by the fouling of sonar dome surfaces. Wood pilings in berthing facilities undergo structural weakening and ultimate destruction due to marine borer and fungal attack. The fouling of piping including steel piping, and bronze couplings and fittings in the sea-water intake piping systems of ship-and-shore facilities, leads to reduced flow rates, valve seat damage, and accelerated metal corrosion. Concrete or ferro-cement or other similar structures are also adversely affected.
It is only since the beginning of this century that improvements have been made in the early Phoenicia methods of using copper cladding and poisonous paints to prevent fouling. Some of these improvements include the use of asphalt as an antifouling coating and coatings using copper or salts thereof. Organometallic salts (i.e. tri-n-butyltin oxide (TBTO), tri-n-butyltin fluoride (TBTF), tri-n-butyltin sulfide (TBTS), being extremely powerful biocides and toxic to a wide range of marine organisms are also components in a variety of antifouling coatings. Investigations into the use of organotin compounds for use in antifouling paints have received additional impetus because coatings with these compounds do not accelerate corrosion and exhibit excellent pigment retention.
These and other present state-of-the-art techniques possess several drawbacks which limit their use as effective methods of antifouling. Asphalt lacks the desired durability to make it an efficient answer to problems posed. Existing antifouling coating systems are paints which contain sufficient water soluble pigments, metal salts and inert fillers for direct contact to occur between the particles within the paint film; as one particle dissolves, another in contact with it is exposed to solvolysis. This process, called leaching is uncontrolled and varies with such factors as coating age, velocity, temperature, salinity, and the primary slime layer. Quantitative information indicates that in most cases the leaching rate is excessive and results in overkill. As such the best available antifouling coatings are inefficient and short lived because of the above mentioned leaching process. This inefficiency leads to the concentration of the toxic metallic components in quantities well above normal oceanic background. Furthermore, it is most important to note that after approximately 12 to 20 months, or 50,000 miles transit the presently used paint systems begin to foul which is indicative of the depletion of most antifouling metal (e.g., cuprous) oxide components from the coating into the marine environment. This short performance time is far less than the 5 year or more life desired for an antifouling coating. Also this depletion of toxic metal compounds into the wake of the ship is responsible for possible detection of ship movements and for pollution.
The leach rate of copper salts and organometallic salts from antifouling coating matrices is governed by the relative proportions and solubilities of three components: rosin, agent salt and pigment (e.g., copper oxides). Rosins are resinous organic acids which have a water solubility of 100 mg/cm.sup.2 /day. In addition to a relatively high solubility, rosins are consumed by sliming marine bacteria. This results in an accelerated biodegradable action, thus adding to the breakdown of the coating and subsequent accelerated release of metallic and organometallic salts. At present the main antifouling salts used by the United States Navy are Cu.sub.2 O (cuprous oxide) (water solubility 0.5 mg/l) and TBTF (water solubility 2.9 mg/l). Leaching of inorganic and organometallic antifouling agent salts from coating formulations could be reduced by using their less water-soluble homologs in conjunction with insoluble pigments and as little rosin as possible.